Clinical Practice Patterns of Anti-Xa Monitoring During Low Molecular Weight Heparin Therapy

Introduction: Therapeutic low molecular weight heparin (LMWH) regimens were developed empirically based on patient weight and renal function without the need for laboratory monitoring. The role of anti-Xa monitoring for patients receiving LMWH has yet to be fully established and is routinely performed only in high risk patients. At our institution, guidelines for monitoring LMWH recommend drawing anti-Xa levels for patients with chronic kidney disease or those with obesity requiring long-term therapy. The purpose of this analysis was to evaluate compliance with institutional guidelines for monitoring anti-Xa levels for patients receiving LMWH, including indication for anti-Xa monitoring, timing of anti-Xa level in relation to drug administration, and follow up actions of providers in response to abnormal anti-Xa levels.

Methods: We conducted a retrospective chart review of adult patients who received therapeutic LMWH (enoxaparin or dalteparin) and were ordered for an anti-Xa level between September 1, 2014 and July 1, 2015. Patients were excluded if they were ordered for an anti-Xa level while on a prophylactic dose of LMWH or while on a non-LMWH factor Xa inhibitor. Data collected included patient demographics, LMWH dose and indication, ordering service, presence of a Hematology consult, reason for anti-Xa monitoring, timing of the level after the previous LMWH dose, and provider response to the level.

Results: One hundred patients met inclusion criteria, 99 of whom received enoxaparin. Venous thromboembolism was the most common reason for LMWH therapy (85%) followed by atrial fibrillation (11%). The most frequent doses of enoxaparin administered were 1 mg/kg every 12 hours (52%), 1 mg/kg every 24 hours (22%) and 1.5 mg/kg every 24 hours (21%). The most frequently specified indications for anti-Xa monitoring were renal dysfunction and acute kidney injury (30%), followed by abnormal body mass index (13%); no indication was specified by the prescriber or pharmacist in 36% of cases. Of the 100 anti-Xa levels in the study, 65 were drawn at the correct time (3-5 hours after the previous dose of enoxaparin); of the 35 drawn at an incorrect time, 33 had levels measured more than 5 hours after the previous enoxaparin dose. Fifty-four of the initial anti-Xa levels were not within the therapeutic range for the dosing interval of LMWH; of these, follow up action was taken for only 21 patients (39%), most frequently resulting in a dose increase or decrease.

Conclusions: In clinical practice, the use of anti-Xa monitoring is inconsistent and commonly done without a documented need.  The majority of anti-Xa levels at our institution are drawn at the appropriate time after the previous dose of LMWH, but action is generally not taken in response to non-therapeutic levels. Additional guidance on the use of LMWH monitoring is needed.